I'm sure that many do, and they probably expect the bag to immediately inflate and presume there's no flow if it doesn't.
At least some airline safety briefings do specifically call out words to the effect of "the bag may not inflate but oxygen is still flowing", but certainly not all. And who pays attention to the safety briefing anyway..?*
* Actually, I do; I take about 80 flights a year but I still pay attention and have a quick look at the safety card. The former out of courtesy to the cabin crew as much as anything, the latter to remind me of some of the peculiarities of the type (if this plane has overwing slides or expects you to slide down the flaps, if all the exits are usable in a water landing or if the rear ones need to stay closed for bouancy, that sort of thing.) Not that it's information I ever hope to need...